An Envelope Elimination and Restoration (EER) Radio Frequency (RF) Power Amplifier (PA) separates the amplitude modulation part, i.e. the envelope, of the RF signal and the phase-modulated constant-amplitude carrier of the RF signal. The envelope signal is amplified and used as the supply voltage to an RF amplifier, whereby the amplitude modulation part being imposed upon the phase-modulated constant-amplitude carrier. In a strict definition of EER lies that the input signal to the final RF amplifier, i.e. the phase-modulated constant-amplitude carrier, has no amplitude modulation. The amplitude variations in the output are therefore produced solely by the modulated supply voltage.
With practical power transistors, i.e. having moderate gain, the amplitude modulation is preferably substantially retained in the input to the RF power amplifier for the sake of efficiency. When the envelope influences the supply voltage to a lesser degree than 100%, the system is called dynamic drain (collector, plate) biasing. Other names for EER and dynamic drain biasing systems are collector (plate, drain) modulation and high-level modulation.
An EER RF PA is theoretically very efficient, since it separates the amplitude modulation part of the RF signal and amplifies this in an efficient, usually switched-mode, class S, base-band amplifier. The amplified envelope is used as the supply voltage to an RF amplifier, whose losses are reduced since the average voltage drop over it is reduced compared to amplifiers with constant supply voltage, always at maximum.
However, the separation of an RF signal, E(t)cos(w(t)) into envelope, E(t), and phase (carrier), cos(w(t)), is a non-linear operation that increases the bandwidth. Both these signals are generally much more wide-band than the incoming RF signal.
The efficiency of an envelope amplifier that has to correctly amplify a signal with large bandwidth is generally low. A narrower bandwidth envelope signal can be amplified with higher efficiency. The problem is that the separated envelope signal typically needs to be represented with a substantially larger bandwidth than the RF input signal to get sufficient output quality.
One solution to this is to let the envelope signal being band-limited by a linear low-pass filter, see e.g. [1]. Such band-limited envelope signal will, however, exhibit overshoots and undershoots compared to the unfiltered version. The occurrence of undershoots and overshoots will generally decrease efficiency and/or signal quality.
Another solution to the envelope amplifier bandwidth-handling problem is to increase the envelope signal for low signal amplitudes, see e.g. [2]. A smooth curve is used, representing the relation between envelope signal and used supply voltage. The curve starts at some fraction of the maximum level and approaches the pure envelope curve at high signal amplitudes. The drawback of this is that the efficiency is much reduced if a significant bandwidth reduction is to be achieved.